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The load–deformation curve (Figure 3.9) is useful for determining the mechanical properties of whole structures, such as an entire bone, or a bone‐implant construct in fracture repair [35].

The initial curved portion is known as the toe region, where low load invokes relatively large deformation, which reflects the uncrimping of collagen fibres in highly collagenous tissues. The linear portion of the curve is called the elastic region, where the object maintains the capacity to return to its original shape once the load is removed. If loading continues through the elastic region to the yield point, then the structure incurs damage. The plastic region of the curve follows the yield point, wherein the material is no longer capable of returning to its original configuration when the load is removed. In the plastic region, the structure deforms to a greater extent for a given load than in the elastic region. If the load continues to increase, the structure will eventually fail. In a clinical setting, the failure point for bone is the load at which it fractures, but in an experimental setting the failure point for a specific biomechanical test may be defined by the investigator. The ultimate load prior to failure is referred to as the ultimate strength of the material. The failure point of bone typically coincides with peak load, as bones have limited ability to deform plastically. The stiffness of the structure is indicated by the slope of the elastic region of the load–displacement curve. The yield, ultimate and failure strengths of a structure correspond to the yield, ultimate and failure load points on the load–deformation curve. The ultimate and failure strength are usually similar in bone but may be different in other materials. Work to fracture (energy absorbed to failure) of a structure is analogous to the material property of toughness and is represented by the area under the load–displacement curve.

Figure 3.9 Representative load–deformation curve for a whole bone.